This invention relates to digital timing circuits in general, and, more particularly, to a more versatile digital timer. Although digital timers are, of course, well known, there are various limitations and problems with the prior timers.
First digital timers respond to a particular line frequency such as 60 hertz and require expensive converters to be operable on 50 hertz line current as found throughout Europe.
The prior art digital timers usually include an internal clock to increment counters at a predetermined clock frequency. Synchronization occurs at the internal clock frequency which is slower than the line current frequency. When the timer circuitry is being utilized to monitor the infusion of a medicine, this present a significant and observable delay between actuation of the timer and commencement of the timing interval, which, of course, occurs only after synchronization has taken place. This delay, of course, is a significant psychological detriment to the patient.
Yet another type of problem which occurs in the prior art timers is related to the mechanism for selecting and establishing the timing interval. Typically, dials or other manual switches are provided on the timer itself for selection of the timing interval. However, in photography for example, there are many instances where an externally generated signal will more accurately establish the timing interval. Thus, during time exposure of photography, a light meter connected to a timer will provide a more accurate signal of the desired timing interval based upon the available light and, furthermore, can provide a variable signal if the available light changes. Furthermore, in the photography dark room, devices which sense the optical density of the negative provide a much more accurate determination of the amount of exposure time necessary to print the picture. Such a device which senses optical density as the photographic enlarger is turned on, may provide an immediate external signal through a timer to determine the duration of the timing or exposure interval.
Another problem with the prior art timers is directed to the number of counters within the timer itself. Obviously, the range of the timer is limited by the number of counters.
Finally, many uses of a timer including both photography and medicine require the availability of a changing display indicative of the elapsed time. For example, if a patient is receiving an infusion of medicine over the three minute interval but during that three minute interval it is necessary for the technician to extract a blood sample, an accurate display of the elapsed time is necessary.
Thus, the invention herein is directed to an improved timer circuit and method of operating a timer circuit to overcome the aforementioned disadvantages of prior art digital timers.